1. Field of the Invention
This present invention relates to mechanical pumps for pumping liquified gases and in particular to pumps adapted for pumping liquified gases in their saturated liquid state.
2. Description of the Prior Art
Cryogenic liquids such as hydrogen, oxygen, nitrogen, argon and liquified hydrocarbons i.e., methane or natural gas, are normally stored and transported in well-insulated low temperature containers to reduce the fluid evaporation losses. Pumps used to transfer such cryogenic fluids between containers or from one container to a point of use are generally mechanical pumps of the reciprocating type. Many conventional cryogenic pumps require the maintenance of a net positive suction head (NPSH), that is, a suction head above zero, to prevent the loss of prime of the pump and/or cavitation. Flow limitations generally result from the maintenance of an NPSH and it is therefore desirable to employ pumps that can operate with a negative suction head or an NPSH below zero.
U.S. Pat. No. 4,239,460 ("'460 Patent") describes a prior art pump which is designed to operate with a NPSH below zero. The '460 pump employs a reciprocating piston which divides a cylindrical housing into a suction and an evacuation chamber. A gas inlet port extends through the side of the housing for channeling liquified gas into the suction chamber. A fixed piston extends from an outlet end of the housing into the evacuation chamber. The fixed piston slides within a cylindrical skirt carried by the reciprocating piston to form a high pressure chamber. The pressurized liquified gas is supplied to an outlet through a passageway within the fixed piston. One way valves control the flow of liquified gas through the inlet, the several chambers and the outlet. While the design of the '460 pump is generally well suited for pumping cryogenic liquids it has several drawbacks. First, the placement of the suction inlet valve and associated suction passageways in the '460 pump limits the achievable ratio of the maximum to minimum volume of the suction chamber. This in turn limits the efficiency of the pump in operating as a compressor in transferring any vaporized liquid (gas) in the suction chamber into the high pressure chamber.
Second, the cool down time of the '460 pump is limited by a gas venting arrangement which allows the free flow of gas to the vent only when the moveable piston is in its forward position.
Third, the '460 pump requires a separate pressure relief valve to vent excess gas in the suction chamber.
There is a need for an improved cryogenic pump which is capable of operating with a sub-zero NPSH.